在A库上建DB_link，更新B库。报ORA-2020 too many database links in use，是要设置OPEN_LINKS和OPEN_LINKS_PER_INSTANCE两个参数，它们的默认值都是4。
alter system set open_links=200 scope=both;
alter system set open_links_per_instance=200 scope=both;
问题来了，是修改A库中的参数还是B库，是修改A库的。
修改这类参数数据库要重启。
Error: ora-02020
Text: too many database links in use
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Cause: The maximum number of active connections to remote databases per user
login has been reached.
Action: If the user has no open cursors, the current SQL statement accesses
more than the maximum allowed remote databases.
Otherwise, the user may free remote database connections by closing
all cursors that access the databases.
If this occurs often, consider increasing the value of the
initialization parameter OPEN_LINKS, which controls the maximum number
of concurrent open connections to remote databases per user process.
OPEN_LINKS specifies the maximum number of concurrent open connections to remote databases in one session.
OPEN_LINKS_PER_INSTANCE specifies the maximum number of migratable open connections globally for each database instance.
